Abstract :
IRSNhascarriedoutastate-of-the-artreviewofthemainexperimentalprogrammesrelatedtofuelbehaviourunderloss-of-coolant-accident(LOCA)conditionsconductedfromthe1970suntilnow,thathasbeensplitinthreeparts.ThesecondpartisdevotedtothequestionofthecoolabilityofblockedregionsinarodbundleafterballooninginaLOCA.Themainfindingsfromthispartarepresentedhere.TheexperimentalcharacteristicsandmainresultsoftheFEBA,SEFLEX,THETIS,ACHILLES,CEGBandFLECHT-SEASETprogrammes,aswellasseveralanalyticaldevelopmentsperformedinassociationwiththeseexperimentalprogrammes,wereexaminedindetailinthisreview.ThecomparisonandcombinationofconclusionsdrawnfromtheseresultsandstudieswereusedtoimproveourunderstandingofthephysicalphenomenagoverningthebehaviourofapartiallyblockedrodarrayduringaLOCArefloodscenario.Ithasalsobeenpossibletodeterminethelimitsofblockagecoolabilityunderthemostseveregeometric(blockageratioandlength)andthermohydraulicconditions.Thus,evenasevereblockageratio(90%)ofamoderatelength(<10cm)doesnotcauseanyparticularproblemsintermsofcoolabilityduringtwo-phasereflood.However,asevereblockagewithconsiderableaxialextension(>15cm)andahighblockageratio(>80%)canlead–underlowrefloodconditions–toasignificantincreaseinblockagesurfacetemperatures,hinderingthefinalcoolabilityofthisblockage.Itisimportanttounderlinethattheseresultswereobtainedinout-of-pileexperimentsperformedwithelectricallyheatedfuelrodsimulatorswithalargegapbetweensimulatorandcladdingbulge,thusnotallowingtosimulatethepossiblefuelaccumulationoccurringincladdingballoons(fuelrelocation),aswasobservedduringallin-piletestswithirradiatedfuelrods.Theimpactoffuelrelocationuponblockagecoolabilitythereforeremainstobeinvestigated.
